This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-202390, filed Jul. 4, 2000, the entire contents of which are incorporated herein by reference.
This invention relates to a process for hydrogenating carbon monoxide. More specifically, this invention relates to a process for producing synthetic fuels having low environmental impact from synthesis gas. In one aspect, this invention concerns a catalyst for use in the hydrogenation of carbon monoxide.
Useful organic chemicals have been produced from carbon resources like petroleum, coal, natural gas and biomass in the following manner. Firstly, synthesis gas, a mixture of carbon monoxide and hydrogen, is produced through the reforming reaction or the coal gasification. The synthesis gas is then allowed to react on specific catalysts at high temperature and high pressure, thus converted to hydrocarbons like alkane and alkene, and oxygenates like alcohol and ether.
These organic chemicals thus obtained can suppress the emission of toxic substances when used as a fuel, since they do not contain sulfur compounds and nitrogen compounds owing to their distinctive manufacturing processes. In particular, methanol, which is mostly produced from the synthesis gas and used as a gasoline additive, has recently received much attention as a hydrogen source for the fuel cell. In the stream of rising environment-conscious, an improved manufacturing method with higher productivity is desired.
In the reaction of synthesis gas, catalysts including metals such as Cu, Fe and Co are generally used. Typical review articles are in the texts xe2x80x9cStudies in surface science and catalysis, vol. 61, NATURAL GAS CONVERSIONxe2x80x9d, A. Holmen et al., Elsevier (1991) and xe2x80x9cStudies in surface science and catalysis, vol. 81, NATURAL GAS CONVERSIONxe2x80x9d, H. E. Curry-Hyde, R. F. Howe, Elsevier (1994).
In spite of their drawbacks of requiring high temperature and high pressure conditions, these catalysts are commercially widely used because of their low costs and availability. However, these catalysts are easily poisoned by various chemical substances in feed gases, particularly by a slight amount of sulfur compounds such as hydrogen sulfide. To avoid this sulfur poisoning, sulfur compounds must be removed to quantity of the order of ppb by installing a desulfurization facility before the reforming or hydrogenation reaction process. Consequently, when the conventional catalysts are used, the manufacturing process becomes complicated and expensive.
Japanese Patent Application KOKAI Publication No. 55-139325 discloses a process for the production of hydrocarbons with sulfur tolerant catalysts having a surface area less than about 100 m2/g and consisting essentially of the metal, oxide or sulfide of Mo, W, Re, Ru, Ni, Pd, Rh, Os, Ir and Pt, and alkali or alkaline earth. In this application, it is noted that a catalyst consisting of MoO3, K2O and carborundum shows no remarkable change in activity (carbon monoxide conversion rate) and gaseous alkene selectivity whether the synthesis gas contains 20 ppm of hydrogen sulfide or not.
Japanese Patent Application KOKAI Publication No. 55-139324 discloses a process for the production of C2-C4 hydrocarbons from the mixture of carbon monoxide and hydrogen with supported catalysts consisting essentially of the metal, oxide or sulfide of Mo, W, Re, Ru, and Pt, and alkali or alkaline earth. According to this application, these catalysts temporarily show low activity when 100 ppm of hydrogen sulfide is introduced into the feed gas, but are regenerated after the feed gas is stopped and hydrogen is fed on them at 500-600xc2x0 C. for one day. It indicates that the catalysts show only low activity in the poisonous atmosphere including sulfur compounds of the quantity of ppm order, and that the feed gas must be once stopped for the contamination of sulfur compounds.
Japanese Patent Application KOKAI Publication No. 61-91139 discloses a method for producing alkene by contacting synthesis gas with a catalyst comprising Mn oxide, alkali metal, sulfur, and Ru. Japanese Patent Application KOKOKU Publication No. 4-51530 discloses a manufacturing method of mixed alcohol with a sulfide catalyst comprising Mo, an alkali promoter, and a support. The latter has the disadvantage of requiring high pressure of at least 7 MPa, usually 10 MPa, for reaction.
As mentioned above, the conventional commercial catalysts for the production of synthetic fuels from synthesis gas are deactivated by sulfur compounds (sulfur poisoning), so that the content of the sulfur compounds must be lowered to the order of ppb before the reaction by means of the upstream desulfurization unit.
On the other hand, aforementioned sulfide catalysts containing Mo or W require high-pressure conditions to achieve proper activity and selectivity.
The object of the present invention is to provide a method of hydrogenating carbon monoxide with high productivity under mild conditions and with simple manufacturing process. It is another object of the present invention to provide sulfide catalysts with high durability, especially excellent sulfur tolerance in the production of synthetic fuels.
According to a first aspect of the present invention, there is provided a method of producing synthetic fuels by hydrogenating carbon monoxide comprising contacting a feed gas containing carbon monoxide and hydrogen with a metal sulfide catalyst comprising at least one element selected from the group consisting of Rh, Pd, Pt, and Hf.
According to a second aspect of the present invention, there is provided a method of producing synthetic fuels by hydrogenating carbon monoxide comprising contacting a feed gas containing carbon monoxide and hydrogen with a catalyst consisting of a solid acid, preferably xcex3-alumina, and a metal sulfide comprising at least one element selected from the group consisting of Rh, Pd, Pt, and Hf.
In the present invention, the feed gas may contain from 1 to 10,000 ppm of sulfur compounds. The molar ratio of hydrogen to carbon monoxide (H2/CO) in the feed gas is preferably within the range from 1 to 5. The feed gas is contacted with the sulfide catalysts preferably at 100-400xc2x0 C. and at 0.1-10 MPa.
The catalyst used in the practice of the invention is a metal sulfide catalyst comprising at least one element selected from the group consisting of Rh, Pd, Pt, and Hf. This metal sulfide catalyst is prepared by sulfiding a metal or a metal compound precursor comprising at least one element selected from the group consisting of Rh, Pd, Pt, and Hf. The sulfiding may be accomplished at the catalyst preparation prior to loading it into the reactor or after loading the precursor into the hydrogenation reaction vessel.
The sulfiding at the catalyst preparation may be carried out by contacting sulfur compounds with the metal; halide such as chloride and bromide; oxide; inorganic salt such as nitrate, phosphate, sulfate, and ammonium salt; organic salt such as acetic salt; carbonyl compound; or chelate. These sulfur compounds include sulfur; alkali metal sulfide such as lithium sulfide, sodium sulfide, and potassium sulfide; ammonium sulfide; carbon disulfide; hydrogen sulfide; and organic sulfide compounds.
The sulfiding after loading the metal or the metal compound precursor into the hydrogenation reaction vessel may be carried out by contacting the metal, halide, oxide, nitrate, or chelate with alkali metal sulfide such as lithium sulfide, sodium sulfide, and potassium sulfide; ammonium sulfide; hydrogen sulfide; etc. The sulfiding may be accomplished by flowing a sulfide compound such as hydrogen sulfide or thiophene with gradually increasing the temperature up to 150-250xc2x0 C. and then to the predetermined operation temperature where temperature is maintained for 1-4 hours.
Besides the aforementioned sulfiding techniques, any conventional method of sulfiding can be used. An example is described in the paper xe2x80x9cHydrodesulfurization Catalysis by Transition Metal Sulfidesxe2x80x9d, T. A. Pecoraro and R. R. Chianelli, Journal of Catalysis, 67, 430-445 (1981). According to this paper, a metal chloride is dissolved in ethyl acetate and lithium sulfide is added with stirring. Then the solution is filtered to yield a metal sulfide. The solid is heat treated in a tube furnace in H2S or H2S/H2 at 400xc2x0 C., cooled to room temperature, washed with acetic acid, filtered, and heated again in H2S or H2S/H2, finally producing the corresponding metal sulfide.
Another example is described in the text, xe2x80x9cJIKKEN KAGAKU KOZA 4th ed. 16. Inorganic Compoundsxe2x80x9d, Chemical Society of Japan, pp. 246-271, or xe2x80x9cRYUKABUTU BINRAN (Handbook of sulfides)xe2x80x9d, SHIN NIPPON TANTYUZO KYOKAI. The latter text describes the most common methods of preparing sulfides as follows:
1. Direct reaction between a metal and sulfur. This method can generate various compositions of sulfides. Depending on the affinity between a metal and sulfur, the reaction is carried out at room temperature (e.g. 2K+S=K2S) or high temperature (e.g. Fe+S=FeS).
2. Reduction of an oxide by sulfur (2CdO+3S=2CdS+SO2, 280-425xc2x0 C.), H2S (La2O3+3H2S=La2S3+3H2O, 1000-1200xc2x0 C.), CS2 (TiO2+CS2=TiS2+CO2, 800xc2x0 C.).
3. Reduction of a sulfate by carbon (Na2SO4+4C=Na2S+4CO), H2 (Li2SO4+4H2=Li2S+4H2O).
4. Reaction between an element and H2S (2Ga+3H2S=Ga2S3+3H2, 800-1250xc2x0 C.)
5. Reaction between a salt and H2S (TiCl4+2H2S=TiS2+4HCl, 600-1000xc2x0 C.)
6. Reaction between a hydroxide and H2S via the formation of an acidic sulfide. (NaOH+H2S=NaHS+H2O, NaHS+NaOH=Na2 S+H2O)
7. Precipitation of an acidic solution by the addition of H2S (sulfides of As, Sb, Sn, Ag, Hg, Pb, Bi, Cu, Cd) and (NH4)2SO4 (sulfides of Zn, Mn, Co, Ni, Fe)
8. Preparation of a low sulfur-content sulfide by the pyrolysis of a polysulfide and by the reaction between a polysulfide and an oxide occasionally in the presence of a reductant; the polysulfide can be prepared by blending a sulfide and sulfur or the reaction between a metal and sulfur in an ammonia solution. (e.g. A polysulfide of an alkali metal can be prepared by the reaction of a hydride and sulfur: 2LiH+3S=Li2S2+H2S.)
The sulfiding can also be carried out by treating a metal compound precursor with sulfur compounds contained in the feed gas in high concentrations during the hydrogenation reaction.
The metal sulfide catalysts in the present invention may contain metals such as Ti, V, Mn, Fe, Co, Zr, and Mo, alkali metal such as Na, K, and Mg, alkaline earth metal, and lanthanoid or actinoid such as La and Th, unless they lessen the effect of the present invention. These materials may be used at the amount from 0.1 to 100 parts by weight of the metal sulfide. The metal sulfide catalysts in the present invention may be used in either bulk or supported form.
The exemplary support materials include inorganic oxides such as silica, alumina, fluorinated alumina, boria, magnesia, titania, zirconia, silica-alumina, alumina-magnesia, alumina-boria, alumina-zirconia, silicoalumino phosphate, and zeolite; clay minerals such as montmorillonite, kaolin, halloysite, bentonite, attapulgite, kaolinite, and nacrite; and carbon. These materials may be used alone or in combination thereof. Although any number of materials can serve as a support, neutral supports such as silica, carbon, titania, and zirconia are preferred, and silica is most preferred. The support may contain nonmetallic elements such as boron and phosphorus.
In preparation of supported catalysts, the supports may be impregnated by techniques known as the wet, dry, and vacuum impregnations.
The preferred amount of loaded metal depends on the property of the support and can not be inclusively determined, but it may be 1-30 mass %, more preferably 5-10 mass % of the catalyst. When this amount is less than the above value, the activity (carbon monoxide conversion rate) per unit of weight of catalyst might be lower. On the other hand, when the amount is greater than the above value, metal sulfide might be agglomerated, so that its activity might be lower.
The sulfide catalyst in the present invention can be used in combination with solid acids. The solid acids include oxides such as alumina, alumina-silica, alumina-boria, alumina-magnesia, and silica-magnesia; zeolites such as X type, Y type, MFI type, and mordenite; and clay minerals such as montmorillonite. xcex3-alumina is most preferred. These solid acids can be used as supports or composites with the sulfide catalysts.
By using the composite catalyst of the solid acid and the metal sulfide, it is possible to produce dimethyl ether (DME) from synthesis gas with a single step process. DME is expected to be a next-generation clean diesel fuel and presently produced with a two-stage process: methanol synthesis and following dehydration reaction.
In the present invention, a feed gas containing carbon monoxide and hydrogen is flown over the sulfide catalyst to be converted into synthetic fuels such as methanol. When the composite catalyst is used, DME can be produced.
The molar ratio of hydrogen to carbon monoxide (H2/CO) in the feed gas is preferably in the range from 1 to 5, more preferably from 1 to 3. This is because (1) the H2/CO molar ratio in the methanol synthesis reaction (CO+2H2=CH3OH) is 2, and (2) the H2/CO molar ratio in the synthesis gas produced from the reforming reaction is usually greater than unity, in most cases with excessive hydrogen.
The feed gas may contain sulfur compounds in addition to carbon monoxide and hydrogen. The content of the sulfur compounds is preferably 1-10,000 ppm, more preferably 100-2500 ppm, most preferably 100-500 ppm.
The temperature of the hydrogenation reaction is preferably 100-400xc2x0 C., more preferably 300-350xc2x0 C. The pressure of the hydrogenation reaction is preferably 0.1-10 MPa, more preferably 1-8 MPa.
According to the present invention, the feed gas containing carbon monoxide and hydrogen is allowed to react on the specific catalyst, so that we can obtain higher activity and higher selectivity under lower pressure conditions. On top of this, a simple or no desulfurization unit is required to treat the feed gas because of the excellent sulfur tolerance of the sulfide catalyst in the present invention. This will simplify the manufacturing process of synthetic fuels.
The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
The present invention is illustrated in more detail by reference to the following examples wherein, unless otherwise indicated, all percentages and ratios are by weight. In the examples, the reaction condition is as follows:
Reactor system: a high-pressure fixed-bed flow reactor
Synthesis gas: 33% carbon monoxide/62% hydrogen/5% argon
Reaction temperature: 240xc2x0 C., 320xc2x0 C, 340xc2x0 C.
Reaction pressure: 5.1 MPa